Advanced Antenna Systems (AASs) is an area where technology has advanced significantly in recent years and where a rapid technology development in the years to come is foreseen. Hence, it is natural to assume that AASs in general and massive Multiple Input Multiple Output (MIMO) transmission and reception in particular will be a cornerstone in a future Fifth Generation (5G) cellular communication system.
Beam-forming has become increasingly popular and capable and, therefore, it is natural to use beam-forming not only for transmission of data but also for transmission of control information. This is one motivation behind the (relatively) new control channel in Long Term Evolution (LTE) known as the enhanced Physical Downlink Control Channel (ePDCCH). When beam-forming is used for the control channel, the cost of transmitting the overhead control information can be reduced due to the increased link budget provided by the additional antenna gain. This is a good property that is also desirable for 5G, perhaps to an even larger degree than what is possible in the current LTE standard.
For downlink Hybrid Automatic Repeat Request (HARQ) transmissions in LTE today, HARQ feedback is sent from the User Equipment device (UE) to the network on either the Physical Uplink Control Channel (PUCCH) or the Physical Uplink Shared Channel (PUSCH), depending on whether the UE has been scheduled for uplink PUSCH transmission or not. The network can thereafter, on an individual HARQ process basis, draw conclusions on whether the last HARQ reception for that process was successful or not (Acknowledgement/Negative Acknowledgement (ACK/NACK)) or even if the downlink assignment reception failed (Discontinuous Transmission (DTX)).
The timing of the transmitted HARQ feedback in LTE is such that, for Frequency Division Duplexing (FDD), the feedback from one HARQ receive process is received in the uplink in subframe n+4 if the corresponding downlink transmission for that HARQ receive process was in subframe n. Thus, the delay between the downlink transmission and the corresponding HARQ feedback is 4 milliseconds (ms) in total. For Time Division Duplexing (TDD), the delay from downlink data transmission to uplink feedback reception may be larger than 4 ms (or equivalently 4 subframes) in order accommodate the half-duplex downlink-uplink split.
For 5G, the HARQ feedback is to be transmitted as part of the Uplink Control Information (UCI) on xPUCCH. As used herein, “xPUCCH” is a term used to refer to the physical uplink control channel in a future generation cellular communications network, e.g. 5G.
The uplink control channel—xPUCCH—may be transmitted on one Orthogonal Frequency Division Multiplexing (OFDM) symbol. This channel will provide a limited number of bits (say, e.g., 1 to 4 information bits) by either: having a number of fixed formats (similar to LTE PUCCH format 1/1a/1b) or having one single format, still allowing for a flexible number of information bits. In regard to using a single format for a flexible number of information bits, performance may possibly be improved with fewer used information bits, since this allows the unused information bits to be used as a short training sequence. Further, it is assumed that there will be an implicit mapping from Downlink Control Information (DCI) Control Channel Elements (CCEs) to UCI CCEs, similar as for LTE.
Existing HARQ techniques are not 100% reliable, are inflexible, and consume a significant amount of resources. As such, there is a need for improved HARQ techniques, particularly ones that are suitable for future generation cellular communications networks such as, for example, a 5G cellular communications network.